The invention relates to a process for monitoring the quality of a strip made of textile fibres which is moved in its longitudinal direction.
In this context a strip made of textile fibres is understood to mean a fibre composite made of fibres which can be used to produce textiles. The fibres are loosely joined to one another therein and form an approximately cylindrical body which is compressible. It is known that strips of this type are present at the output of card machines, combers or drawing frames and are stored temporarily in cans. Yarn is preferably produced from strips of this type.
The quality requirements on strips are constantly increasing. Up until now on-line detection of CV% values, spectrograms, thick points and variations in the strip weight over several metres of the strip length etc. was sufficient. Therefore, in the last few decades it has been possible to considerably improve the mean linear uniformity of yarn and drawing frame strips. Consequently, the quality of end products such as woven and knitted fabrics has also improved. In these improved end products rare occurrences such as thin and thick points in the yarn, are all the more disruptive the longer they are. The optical appearance of flat goods produced today usually appears so uniform that rare thin and thick points in the yarn, which owing to the surrounding increased yarn non-uniformity did not lead to complaints up until a few years ago, are classified as disruptive. That which was still regarded as best quality several years ago would now already be considered in the range of second quality, with corresponding losses in yield.
More recent spinning processes require more extensive detection of quality features of the strip in order to be able to finely tune the spinning machine such that yarn production without interruption is achieved.
With known drawing frame levelling, short thick and thin points in the strip may no longer be levelled if they are only a few cm in length as levelling only starts to become effective after approximately 20 cm strip length. It should be noted that even relatively short flaws in the strip, which are still present at the output of the frame, are often lengthened in the subsequent process stages by a factor corresponding to the overall drawing in this process stage. Therefore, flaws in the strip of several centimetres in length produce flaws in the yarn of several metres in length. Thick points in the strip are potentially only partially drawn and removed. Thick points in the form of transferred bundles of fibres, for example, may no longer be drawn. Therefore, with increasing overall draw the relative proportion of such a thick point will also increase and its disruptive effect is made disproportionately noticeable. With thin points in the strip which are lengthened by the overall draw, the thickness of the product also decreases in this region accordingly. Rovings drawn in this way can lead to disruptions during spinning or can leave behind visible traces in the end product, so the end product is downgraded with regard to its quality class. This leads to losses in yield as a result.
A process and a device for monitoring the quality on-line in the spinning mill advance unit is known from EP 0 606 615. Thick points in a strip can be detected thereby, so an alarm is triggered or the plant can be stopped. To this end, the mass of the strip is measured and a signal derived from this measurement compared with a limiting value. This limiting value is determined from the mass non-uniformity (CV%) and a limiting value factor.
A disadvantage of the known device or known process is that thick points of short length are not fully detected and taken into account for an alarm etc. Thick points are also only taken into account independently of their length. It is sufficient to detect a specific variation in the mass, based on a predetermined unit of length, in order to influence, for example, a control process.
Measurement of the strip mass at the output of a frame is known from WO 93/18213. The measured strip mass is used to control the variable draw in the frame. Measurement of the strip mass forms part of a closed control circuit for the frame, in this case.
A particular disadvantage of this device is that the length of a variation in the mass from a mean value is significant for this control in that it is present for a certain period of time and therefore influences the actual draw ratio during this time. However, there is no combined assessment of a variation in which the size of the variation in the mass and the length or duration of this variation are calculated simultaneously with regard to the effective influence in the end product.
An object of the invention is therefore to create a device and a process which allow flaws in a strip to be dealt with in a targeted manner and allow the production of a strip which is as free as possible of variations in the cross-section or mass.
This is achieved in that variations in the mass of the strip are continuously detected at the output of a machine for spinning preparation, such as in particular a card machine, comber or a drawing frame, and are converted into an electrical signal after which this signal is compared with a plurality of predetermined limiting values. Limiting values are provided for variations transverse to the longitudinal direction and limiting values are provided for the extent of the variation in the longitudinal direction of the strip. A length and a cross-sectional variation are determined for each variation from the comparison of the signal with the various limiting values and a quality feature is formed from the length and the cross-sectional variation, identical quality features being counted. This corresponds to a classification of thick and thin points in the strip and the association of the detected variations to a group or to a group of classes is designated as a quality feature. The limiting values for the variation in the longitudinal direction are, for example, at intervals of 1 cm to one another and the limiting values for the variation transverse to the longitudinal direction at intervals of 5% of the mean mass of the strip. A measure for improving the quality of the strip is derived from the quality features, i.e. from the occurrences or variations counted in the various classes. Such a measure is the emission of a warning signal, for example by actuating a warning light, stopping production machines which have caused or have not reduced the variation, or of production machines which are to process the strip with the inadmissible variation and finally, cleaning of the relevant processing machine. The flawed portion of the strip can be removed and analysed from which information can be obtained as to how settings in the relevant production machines are to be altered. It can also be seen from the classification whether a flaw or a variation deserves further observance at all and triggers measures or not. A prediction regarding consequences in the subsequent processes can be made. For example, it can be assumed that, starting from certain classifications, broken threads during spinning can be predicted. Therefore, the performance in subsequent processes can be estimated.
The advantages achieved thereby can, in particular, be seen in that the flaws or variations in the strip can be dealt with in a very differentiated manner. Every user can draw specific conclusions valid for his production plant from the signals or from their classification and take suitable measures. Flaws in the strip are detected early and can be dealt with where they occur according to their nature, or it can be detected early on that they cannot be corrected. Therefore, the corresponding portion of the strip or a part of the raw material can be removed in a production stage which permits this with relatively few or harmless interventions. Therefore it is, for example, easier to remove flaws from the non-woven fabric of a card machine than from a strip.